1. Field of the Invention
The present invention relates to the field of semiconductor and micromechanical devices and processing techniques therefor, and particularly to resonance sensors, and to the manufacture of resonating structures for such sensors for pressure measurements.
2. Description of Related Art
Ultra miniaturized sensors for minimal invasive use have become important tools in for example heart surgery and diagnosis during the last ten years. So far optical or piezoresistive principles have been employed and sensors based on these technologies are commercially available. However, the known sensors suffer from some drawbacks. In particular calibration is fairly complicated and also the manufacture is not an easy task with fairly high rejection rates of finished products. Therefore there is a search for new types of sensors and other means and methods of making ultra miniaturized sensors in an efficient and economic way.
In U.S. Pat. No. 5,747,705 (Herb et al) there is disclosed a method of making a thin film resonant microbeam structure, and in U.S. Pat. No. 5,808,210 (ibid) there is disclosed such a resonant microbeam sensor.
By performing this known method one obtains a resulting device usable as a sensor for pressure and/or temperature measurements. It comprises a substrate, a cavity formed in the substrate usable as a reference cavity, a composite diaphragm structure covering said reference cavity, wherein the diaphragm comprises two layers. A first layer of said composite diaphragm forming a top wall or cover of said reference cavity is provided with at least two parallel slits, such that a resonating portion of said layer is formed. This resonating portion, which is referred to as a "microbeam", is an integral part of said first layer. There is also provided a second layer of said diaphragm, deposited on top of said first layer, but with provisions made for forming a small cavity directly above this resonating portion.
However, there are certain limitations to this structure, in particular the fact that the beam is a part of the composite diaphragm. Thus, the entire diaphragm structure will necessarily always be thicker than the beam. This will adversely affect the sensitivity of the sensor.
U.S. Pat. No. 5,090,254 and U.S. Pat. No. 5,188,983 (both Guckel et al) relate to polysilicon resonating beam force transducers and the manufacture thereof. A transducer according to these disclosures comprises a substrate, a cavity formed in the substrate, a polysilicon beam firmly mounted to the substrate, and an encapsulating shell surrounding the beam. Pressure is measured by providing the substrate as a diaphragm of a pressure sensor and subjecting the substrate to the environment where pressure is desirable to be measured. The transducers obtainable by the teachings therein suffer from a limitation in that they cannot be sufficiently miniaturized for certain applications, such as a pressure sensor for use in coronary vessels. In particular it is explicitly said that the beams can be made to thicknesses in the range "of a few microns". For the mentioned applications this is by far too thick, and it would be desirable to have access to beams exhibiting a thickness of 1 .mu.m or even less, say 0.5 .mu.m. Furthermore, if the force transducer as disclosed in this patent should be applied for pressure measurement, in fact what is referred to as the "substrate" in this patent, would have to be used as a flexible diaphragm. This would necessarily mean that the components of the transducer would have to be miniaturized far beyond what is reasonable. In order to make such a structure, if indeed at all possible, back side etching and a complicated sealing technique would have to be used.
In U.S. Pat. No. 4,884,450 there is disclosed a pressure sensor for use in gas or oil well logging. It includes a resonator element mounted on an optical fiber ferrule. The resonator is located in an evacuated cavity and is mounted on a flexible diaphragm, whereby pressure changes may be detected as changes in the resonant frequency.
This disclosure teaches a resonator member in the form of a single crystalline silicon structure. The resonator structure is made as a separate unit by selective etching from a body of single crystal silicon. The diaphragm/beam assembly is mounted to a substrate, actually a ferrule incorporating an optical fiber for excitation and signal transmission purposes, via a peripheral flange e.g. by electrostatic bonding. This substrate is made of boro-silicate glass. Thus, the entire device is made by assembling two separate parts, the diaphragm/beam unit and the substrate (ferrule). This is a very difficult operation if one wants to achieve a proper vacuum, as required for adequate resonance of the beam to occur. Especially if it is desired to make the sensor in the dimensions necessary for employment in e.g. a coronary vessel or other locations of similar size. Another drawback with this device is to be found in the selection of materials. Glass in general degasses, i.e. releases gaseous substances into the surrounding, at temperatures of about 400.degree. C., normally necessary to attach glass to silicon.